Under current MOSFET transistor technology, raised source/drain regions are used in order to provide a transistor having ultra-shallow junctions. The raised source/drain region gives rise to numerous problems. A facet is normally formed at the interface where the raised source/drain region abuts with the insulating field oxide of the transistor structure. Additionally, a facet is normally formed at the interface where the raised source/drain region abuts with the sidewall spacing insulator adjacent the transistor gate. Subsequent silicide processes may create a spike at the location of either facet, and the spike may penetrate into the underlying semiconductor substrate and through the shallow junction. This spike may therefore short-circuit the source/drain to the silicon substrate.
In the construction of raised source/drain transistors, it is important to control diffusion associated with dopants used to create the shallow junctions of the transistor. Typically, a first implant stage is performed prior to the deposition of the raised source/drain region in order to form an electrical connection underneath a thick sidewall spacing insulator, thereby coupling the moat to the channel region. However, the subsequent step of depositing the raised source/drain region may cause the previously disposed dopant to further diffuse to such an extent that the operating characteristics of the device are changed.
Another problem associated with current raised source/drain transistors may occur during the formation of silicided contact areas. If the gate and source/drain regions are not properly isolated from one another, the silicided process applied thereto may create a conductive short-circuit between the gate and either the source or drain. Further, the unintended formation of a silicide between gate and source/drain may make it difficult to subsequently etch the silicided region in order to isolate the gate from either the source or drain.
Therefore, a need has arisen for a raised source/drain transistor which eliminates problems associated with spike formation, uncontrollable movement of dopants, and difficulties in electrically isolating the gate from the source/drain regions.